やる, やるな, and やるなぁ~

Of course, やる is the same as する, which means to do.
やるな means "don't do that," as placing a な after a verb in its plain form is a strong way to tell someone not to do whatever action the verb denotes.

やるなぁ~ means something along the lines of "good job," "well done," etc.
I just learned the third of these, sort of by accident.  I was watching a TV show called Hammer Session, and I heard one of the characters say it but didn't quite understand it.  The subtitles translated it as "Aren't you good?"  I couldn't understand why やる was used in that situation.  Something like 「やった！」 (I did it!) I could see, but やる?

Two days later, I was at my desk in the teachers' room at school, and I overheard one of my Japanese colleagues say 「やるなぁ~」.  My head perked up.  I asked her what exactly she meant just now, and she explained the differences between the three usages of this word.  I went back to the video that night to listen to it again.  It turns out I misheard the actress.  She doesn't say  「やるなぁ~」; she says 「やるじゃん!」。
I think it means the same thing, though.

There's no embedding code on the video, but if you want to see it the url is:
http://www.drama.net/m1/hammer-session-episode-4/part3
 As the link tells you, it's Episode 4, part 3, of Hammer Session.